1. Field of the Invention
The present invention relates to an exposure control device for use in an electrophotographic printing apparatus, such as a laser printer or a laser copying machine and, more particularly, to an exposure control device for controlling a laser light source for emitting a laser beam to expose a photosensitive image carrier.
2. Description of the Related Art
A typical electrophotographic printing apparatus will be described, taking, as an example, a laser printer equipped with a photosensitive drum serving as an image carrier. The laser printer uniformly charges the surface of a photosensitive drum rotating on a shaft in one direction and selectively exposes the charged surface of the drum by using a focused laser beam, thereby forming an electrostatic latent image on the photosensitive drum. The latent image is developed by supplying the photosensitive drum with a developer which selectively adheres to the latent image, so that a visible developer image is formed. The visible image is then transferred from the drum to paper. Such a laser printer is disclosed in the Japanese Unexamined Patent Disclosure Hei 1-234813.
The laser printer of this disclosure has a laser optical system for exposing the charged surface of the photosensitive drum. The optical system has a laser light source for emitting a laser beam. The laser beam is emitted to a polygon mirror rotated by a mirror motor, and then reflected on the polygon mirror to a predetermined scanning range. Each reflecting plane of the polygon mirror changes the direction of reflection of the incident laser beam according to its angle of rotation, thereby permitting the laser beam to scan the scanning range horizontally. The scanning range comprises an exposure area corresponding to the span of the drum surface and a non-exposure area outside the exposure area. A reflecting mirror receives the laser beam scanning the exposure area and reflects the received beam to the drum surface located therebelow. The drum surface is scanned by the laser beam in the direction of the main scanning which is parallel to the drum shaft. Further, the laser optical system has a pickup mirror and a photo sensor in order to identify a scanning position. The pickup mirror mirror is set at a position of the non-exposure area adjacent to the exposure starting end of the exposure area and reflects a laser beam reflected from the polygon mirror toward the photo sensor. The photo sensor converts the laser beam reflected from the pickup mirror to an electric signal.
The laser power of the laser beam, is preset by means of a laser control voltage to obtain a desired printing tone. However, the laser power may change, due to change in the temperature of the laser light source which depends on the frequency at which the laser sight source is driven. Therefore, the laser printer has a laser power detector formed in the laser light source, and corrects the driving current supplied to the laser light source based on the output voltage therefrom so that the laser power is stabilized at the preset level.
However, the above-described laser printer suffers from the following drawbacks because it must be controlled so that the laser power is kept constant all the time. Suppose that characters drawn by bold lines, such as Gothic characters, are printed. In this case, there is a fear that a slight increase of the printing tone makes the bold lines come in contact with each other and thus characters are defaced. In order to avoid such a situation, it is required to set the laser control voltage to 2 volts or less. However, the photo sensor cannot sense a laser beam with the laser power depending on such setting. In order to obtain the laser power of a laser beam which can be sensed with certainty, it is required to set the laser control voltage to 3V or more. If the laser printer is connected to a CAD (computer aided design) apparatus to draw precision graphic forms in accordance with image data therefrom, the laser control voltage must be set from 8V to 10V, in order to draw the graphic forms clearly. If, however, the laser control voltage are set to such a high voltage, the photo sensor will sense not only a laser beam reflected from a desired spot in the pickup mirror but also laser beams reflected from the periphery of the spot. This will cause the output signal of the photo sensor which should be rectangular in waveshape to become dull, resulting in a lag in the start timing of exposure.